Abstract
Abstract. We estimated the black carbon (BC) concentration over the Hindu Kush Himalayan region (HKH), its impact on snow albedo reduction, and sensitivity on annual glacier runoff over the identified glaciers. These estimates were based on free-running aerosol simulations (freesimu) and constrained aerosol simulations (constrsimu) from an atmospheric general circulation model, combined with numerical simulations of a glacial mass balance model. BC concentration estimated from freesimu performed better over higher altitude (HA) HKH stations than that over lower altitude (LA) stations. The estimates from constrsimu mirrored the measurements well when implemented for LA stations. Estimates of the spatial distribution of BC concentration in the snowpack (BCc) over the HKH region led to identifying a hot-spot zone located around Manora Peak. Among glaciers over this zone, BCc (>60 µg kg−1) and BC-induced snow albedo reduction (≈5 %) were estimated explicitly being high during the pre-monsoon for Pindari, Poting, Chorabari, and Gangotri glaciers (which are major sources of fresh water for the Indian subcontinent). The rate of increase of BCc in recent years (i.e., over the period 1961–2010) was, however, estimated to be the highest for the Zemu Glacier. Sensitivity analysis with a glacial mass balance model indicated the increase in annual runoff from debris-free glacier areas due to BC-induced snow albedo reduction (SAR) corresponding to the BCc estimated for the HKH glaciers was 4 %–18 %, with the highest being for the Milam and Pindari glaciers. The rate of increase in annual glacier runoff per unit BC-induced percentage SAR was specifically high for Milam, Pindari, and Sankalpa glaciers. The source-specific contribution to atmospheric BC aerosols by emission sources led to identifying the potential emission source being primarily from the biofuel combustion in the Indo-Gangetic Plain south of 30∘ N, but also from open burning in a more remote region north of 30∘ N.
Highlights
Aerosols are particles in the atmosphere known to impact Earth’s climate directly as well as indirectly (Foster, 2007)
At lower altitude (LA) stations, we evaluate the estimates from constrsimu with measurements
Compared to ratio approach (RA), the profile approach (PA) estimates exhibit a better coherence with the measured values, with these estimates amounting to 90 %–100 % of measured data and a normalized bias of −14 % to 13 %
Summary
Aerosols are particles in the atmosphere known to impact Earth’s climate directly as well as indirectly (Foster, 2007). Black carbon impacts the climate through a direct effect by absorbing sunlight (Ramanathan and Carmichael, 2008) and an indirect effect through cloud alterations in precipitation efficiency (Lohmann and Feichter, 2005). Based on the atmospheric BC measurements at stations (e.g., Nepal Climate Observatory-Pyramid, NCO-P, Hanle) over the Hindu Kush Himalayan (HKH) region, radiative forcing due to BC and the BC-induced snow albedo reduction (SAR) for estimated BC concentrations in snow over the stations was reported in recent studies (Nair et al, 2013; Yasunari et al, 2010). The ability of coarse-gridded models to adequately simulate the snow depth and thereby the BC concentration in snow and atmospheric BC radiative forcing is limited (Menon et al, 2010; Ménégoz et al, 2014; Qian et al, 2015)
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